Switched power converters are usually used in power supplies and are implemented in two typical forms. The first implementation is a Switched-Inductor Converter (SIC), in which the component that accumulates the energy during conversion is an inductor. SICs are widely used in high power applications since they have a wide operating range with high efficiency and hence, the efficiency is not dramatically affected by the conversion ratio. However, SICs have relatively large dimensions and they cannot be used in systems where size reduction is critical.
The second implementation is a Switched Capacitor Converter (SCC), in which the component that accumulates the energy during conversion is a capacitor. SCCs are widely used in lower power applications where size reduction is critical, since they are relatively compact. However, SCCs have high efficiency only in a single operating point or at several operating points (which correspond to discrete conversion ratios), depending on their design.
Another problem with conventional SCCs is the fact that they have limited capabilities for voltage regulation due to the tight relationship between the voltage gain and the converter efficiency. In such SCC converters, the efficiency is tied to the ratio between the output voltage Vo, and the target voltage VT (which is the SCC's output voltage with no load), which stems from the rigid proportionality between the input and output charges.
                    η        =                              V            0                                V            T                                              (                  Eq          .                                          ⁢          1                )            
Regulation can be obtained either by varying the SCC parameters, i.e. by adding losses, or inserting a post regulation stage, in order to match the required conversion ratio.
A more advanced approach for voltage regulation by SCC is to generate multiple conversion ratios and therefore increase the effective operation range. However, the system efficiency would remain of a discrete nature. The multiple conversion ratios approach has shown advancement in the utilization of SCC, in particular as a high efficiency first stage converter that may be followed by a reduced size local regulator.
Resonant SCC operation with Zero Current Switching (ZCS) has been used to reduce the switching losses, while allowing higher switching frequency operation and thereby, potentially reducing the total volume of the converter. However, even with ZCS implementation, high efficiency is still obtained only for discrete conversion ratios.
Another problem in existing soft-switched SCCs, which should create an output voltage that is different from the target voltage, is the fact that the charge-balance of the flying capacitor(s) after a charge/discharge cycle is not zero, due to the residual charge left in the flying capacitor(s). This residual charge prevents the system from converging to the desired output voltage by increasing or decreasing the output voltage, in order to satisfy the charge-balance of all the capacitors. The result will be a drift of the output voltage from the desired operation point back toward VT.
FIG. 1 (prior art) schematically shows a conventional 1:1 resonant switched capacitor converter. The resistors Rs1 and Rs2 represent loop series resistances, where V1 and V2 are the input and output voltages, respectively.
FIG. 2 (prior art) schematically shows typical waveforms of the flying capacitor C in the SCC described in FIG. 1 (prior art) for a case that V1≠V2. The dotted line shows the capacitor's current, Ic, while the solid line shows the capacitor's voltage Vc. The dashed line shows the average output current Io which corresponds to each switching cycle. The current waveform shows that although ZCS is obtained, the charge received from the source is not equal to the charge delivered to the output. This entails an unbalanced capacitor voltage (i.e., the starting point is not equal to the end point in each cycle) that continues to rise in every cycle, thereby leading to a runaway effect, which must be compensated.
“Zero voltage switching double-wing multilevel modular switched-capacitor DC-DC converter with voltage regulation,” (C. Dong, L. Xi, Y. Xianhao, F. Z. Peng, IEEE APEC, 2013, No., pp. 2029-2036 discloses a method to solve the problem of residual charge of the capacitor and allows regulation by introducing series losses. However, this approach reduces the overall efficiency of the converter.
“Analysis of Step-down Resonant Switched Capacitor Converter with Sneak Circuit State” (Qiu Dongyuan and Zhang Bo, 37th IEEE PESC 2006), pp. 1-5 discloses a topology (named “Sneak Circuit State”) with an additional switching stage to internally circulate the charge, to emphasize an inherent feature of the original resonant SCC configuration.
“A resonant switched-capacitor converter for voltage balancing of series-connected capacitors” (K. Sano and H. Fujita, 2009 International Conference on Power Elect. and Drive Systems, pp. 683-688) discloses circuitry to circulate the charge. In this case, the operation of the converter was set above the resonant frequency, thereby exhibiting inductive behavior. This allows reversing the inductor current using phase shift control, which also regulates the power flow direction. However, according to this solution, soft switching cannot be guaranteed for the entire operation range.
“Unified Analysis of Switched-Resonator Converters” (M. Jabbari, IEEE Trans. on Power Elect. 2011, vol. 26, no. 5) discloses a solution which combines resonant and linear operation to completely discharge the energy of the LC tank in every switching cycle. However, the direction of the power flow is still dictated either by the system configuration or by the source of the higher potential.
All the conventional methods still have at least some of the disadvantages of reducing the overall efficiency of the converter, soft switching that cannot be guaranteed for the entire operation range, limited direction of the power flow, relatively large size or dependency of the target voltage from the conversion ratio.
It is therefore an object of the present invention to provide a high efficiency resonant Switched Capacitor Converter (SCC) which overcomes the problems of prior art converters and has a wide operating rage, similar to SICs.
It is another object of the present invention to provide a high efficiency resonant Switched Capacitor Converter (SCC), which allows eliminating the dependency between efficiency and conversion ratio.
It is a further object of the present invention to provide a resonant Switched Capacitor Converter (SCC), with high efficiency over a wide conversion ratio.
It is still another object of the present invention to provide a high efficiency resonant Switched Capacitor Converter (SCC), which allows obtaining improved flying capacitor's charge-balance.
Other objects and advantages of the invention will become apparent as the description proceeds.